Year

1987

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

In this work, primary data are presented in order to characterise the precise numerical variation of all the important components of molecular interaction potential functions. Despite access to very limited computing facilities this work documents the current state of the art capacity of applied quantum theory to represent interactions between the two electron systems He, H2 and Li+.

Accurate ab-initio data is presented on the He-He, He-H2 and H2-H2 van der Waals interactions and the He-Li+ ion-induced dipole interaction.

Ab-initio studies necessitate the use of various approximations, due to the constraints of computer time available to the investigator. Careful consideration of the approximations used at every stage of the investigation and of their effects on the final potential function needs to be given.

Approximations inherent in these studies are manifested in the computational formalism used to carry out the electronic structure calculations, the basis set used to describe the molecular orbitals, the number of conformations used to characterise any anisotropic interaction and the method used to correct for the effects of basis set superposition errors. To obtain a reliable description of a potential function it is important to select an optimal balance in the degree of completeness of the three major areas above and to minimise and properly correct for the effects of basis set superposition errors.

In the present work the CEPA2 formalism was used to carry out the calculations. It is a computationally efficient method which accounts for most of the electron correlation energy of closed shell ground state systems. The computational efficiency enables the use of large basis sets so that the various energy terms contributing to an interaction can be modelled correctly.

The potential functions examined were chosen because they are the benchmark interactions of chemical physics and have been the focus of much experimental study. They have few electrons and thus lend themselves to the most accurate ab-initio treatments. The van der Waals interactions are also of considerable astrophysical interest. The interactions explicitly studied in this work are (i) He-He, (ii) He-H2, (iii) H2-H2 and (iv) He-Li+.

The attractive intercorrelation, or dispersion energy term of the van der Waals region of the systems studied will suffer to varying extents due to the limitations of the CEPA2 formalism. Thus in regions of the potential functions where this term dominates, the interaction energy will be systematically underestimated to a small degree. These interactions were nevertheless studied because no previous study has been able to give a better theoretical description of this region of the potential functions. The repulsive regions of these potential functions as well as the intracorrelation energy term will receive the best ab-initio descriptions to date primarily because of the very large and flexible basis sets utilised for each interaction studied.

For the He-Li+ study the present potential is definitive.

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